Nysalorean Illumination is associated with the Solar religion of Peloria, and perhaps we should think about how the Bright God has embraced Nysalor.
Yelm is one of the Greater Gods and a key upholder of the Cosmic Compromise. As the Ruler of the Universe, Yelm must interact with the other forces of the cosmos. This includes the gross and unclean material elements of the cosmos.
Lodril dives into the filth and revels in pollution. He has lost his connection to the Light entirely. Because of this, he is destined to be the servant, even the slave, of others. No sensible person trusts Lodril to do anything but wallow in pollution – unless Lodril is forced to do otherwise through coercion and command.
Dayzatar remains pure through complete abstention from all pollution. He is the Pure Light, uncompromising and uncompromised. Dayzatar does not interact with the rest of the world.
Yelm must interact with the rest of the world, but is not corrupted by it because of his supreme moral virtues although he is not Pure. In short, because Yelm is Yelm, he can interact with Lodril’s world without becoming Lodril. He is not Pure like his elder brother, but he is not polluted.
We can see this dynamic with Third Age Nysaloreanism. Nobody sane lets peasants get Illuminated – only bad things can happen. That gets us Wants More, Make Me A Duke, Margins, or even Black Arkat.
Dayzatar is able to become Illuminated but still refrains from interaction with the polluted world – including Chaos. He is freed from fear of Chaos, but still recognizes is as pollution and remains Pure.
Yelm is Illuminated and can deal with the polluted world without become corrupt through his supreme moral virtue. This includes Chaos of course, for Chaos is part of the universe. But because we are Yelm we cannot be tempted by Wakboth.
And with that, Tatius the Bright becomes completely understandable.